In the field of mass spectrometry using an ion trap mass spectrometer or other apparatuses, a technique called the MS/MS analysis (tandem analysis) is commonly known. In a typical MS/MS (=MS2) analysis, an ion having a specific mass-to-charge ratio (m/z) of interest is selected as a precursor ion from an object to be analyzed. The selected precursor ion is dissociated by collision induced dissociation (CID) to produce one or a plurality of product ions. The pattern of dissociation depends on the structure of the original compound. Accordingly, it is possible to identify the target compound and/or grasp its chemical structure by performing a mass spectrometry of the product ions produced by the dissociation and analyzing the thereby obtained MS2 spectrum. If the ion cannot be dissociated into sufficiently small mass-to-charge ratios by only one stage of the dissociating operation, an MSn analysis may be performed, in which the dissociating operation is repeated a plurality of times, and the eventually obtained fragment ions are subjected to a mass spectrometry.
In a molecule identification method described in Patent Document 1, in the process of identifying an unknown compound or deducing its chemical structure from data obtained by the aforementioned MSn analysis (MSn spectrum data), a database search is performed with reference to a database (or library) in which spectrum patterns, fragment structures and other information are previously registered. However, to use such a technique, a database of MSn spectra must be prepared beforehand.
In recent years, liquid chromatograph mass spectrometers (LC/MSs) consisting of a liquid chromatograph (LC) coupled with an MS2 (or MSn) mass spectrometer have been commercially available in large numbers and are widely used in various fields. However, the amount of MSn spectrum databases for such systems is far from adequate. One of the reasons for this situation is that LC/MS is capable of observing an enormous number of molecular species (several millions) and it is difficult to create an MSn spectrum database which exhaustively covers such an enormous number of molecular species. Another reason for the difficulty in creating the database is that, in a measurement by LC/MS, even if the substance is the same, the pattern of dissociation easily changes depending on the analyzing conditions (e.g. the type of mobile phase in the LC, the ionization method, the ionizing conditions or the CID conditions) as well as the system configuration, which leads to a significant difference in the peak pattern of the MSn spectrum.
Due to such reasons, identifying a substance using a database search for MSn spectra has been difficult for LC/MS, and especially for a system using an MSn mass spectrometer. Even if such identification is possible, the kinds of identifiable substances are considerably limited. Thus, in an MSn analysis using an LC/MS, the database search for an MSn spectrum has been practically unavailable for the identification of a completely unknown substance.